Introduction

The growth of any community is hinged on the availability of basic amenities such as water, good road network and electricity. The search for sustainable, clean and portable water is a struggle that will never end as it aids in the growth of any community (Salako et al. 2009).

Niger State in north central Nigeria experiences an annual rainfall which ranges from 1200 mm to 1600 mm from the southern part of the state to the northern region. The duration of the rainy season ranges from 120 to 150 days or more from the north to the south (Baimba 1978). The amount of rainfall is usually limited to few months within the year, such that water from surface sources cannot meet the demand for development (Baimba 1978). This inadequacy and pollution of surface water brings about the overdependence on groundwater from hand dug well and borehole either for individual or public consumption.

Mapping of the subsurface structure and interpretation has provided information about the geologic formations and physical properties of the geologic materials (George et al. 2015a, b). In groundwater exploration, the knowledge of the aquifer characteristics is important as it helps in determination of natural flow of water, depth to bedrock, availability, quantity and quality of the groundwater. Since water occurs both as surface water and groundwater thus forming the hydrological cycle (a continuous re-circulatory movement of waters of the earth).

Groundwater is the water in porous rocks beneath the water table. In other words, it is the water that is contained in aquifers. An aquifer is any geologically deposited material that has the ability to store and transmit significant quantity of water. Groundwater contains dissolved ions and allows current to flow through it due to the conductive nature of the ions. Permeability, porosity, resistivity, thickness of the layers and aquifer yield are subsurface properties that play important role in groundwater movement, availability and potential. Aquifers are found deep beneath the earth’s surface, and due to their depth of storage and natural filtration through the different subsurface layers (soil horizons), groundwater is relatively pure and grossly protected from surface pollutants. Groundwater is a source of uncontaminated water and is very important for our daily needs like drinking and even agricultural purposes. Groundwater is of major importance to civilization because it is the largest reserve of drinkable water in the regions where humans live. Groundwater may appear at the surface in the form of springs, or it may be tapped by wells/boreholes (Salako et al. 2009). Therefore, to have a sustainable groundwater development for our needs, the study of groundwater potentials becomes very important.

The subsurface is made up of different geologic materials occurring at varying depths. In a basement complex terrain, the lithological unit that can be observed comprises of the weathered layer, weathered/fracture basement and fresh basement (Omorinbola 1984). The porosity and permeability capacity of this subsurface lithology depends on the type of geologic material occupying them (Ojo and Olorunfemi, 1990). Weathered layer that consists of clay would have less porosity, while a highly weathered/fracture basement would be highly porous and permeable (Hazell 1992).

Geology of the study area

The study area is located at Paggo village in Minna, Niger State. It lies in north central Nigerian Basement Complex with latitude 9°271N and 9°281N and longitude 6°381E and 6°391E. The mean annual temperature is 22–25 °C. The area is characterized by both dry and rainy seasons. It has an average rainfall of 1200–1300 mm. The vegetation covering this area is Guinea savannah which is characterized by tall grasses, shrubs and also sparsely distributed trees. Dry season is usually accompanied by dust and fogs. The dry season usually last within the month of November to February followed by the rainy season from April to October. Most of the rocks are granite, gneiss and quartzite. Niger state has two main rock formations like most state that is on the same latitudes which are sedimentary and basement complex rocks (Amadi et al. 2012) (Fig. 1).

Fig. 1
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Geological map of Niger state showing basement complex and sedimentary basin (Amadi et al. 2012)

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Materials and method of the study

The data were acquired with the ABEM SAS 4000 Terrameter, Global Positioning System (GPS) for taking accurate coordinate of the VES point and elevations, metal electrodes, measuring tape, labelled tag (used in locating station position), hammer (used in driving the electrodes into the ground). The Schlumberger array was adopted. The electrode spread of AB/2 was varied from 1 to a maximum of 100 m. The electrical resistances obtained were multiplied by the corresponding geometric factor (k) for each electrode separation to obtain the apparent resistivity. The apparent resistivity was computed using Eq. (1). Sounding data were presented as sounding curves, by plotting apparent resistivity against AB/2. The IPI2win software was then used to obtain the n-layer model curve for the Schlumberger sounding curves. This software automatically interprets the Schlumberger sounding curves. The plotted curves reveal the number of layers, thickness, depth and the average resistivity for each layer at different VES points automatically.

$$\rho_{\text{a}} = KR$$
(1)

where ρa is an apparent resistivity and the earth resistance (R) is given as

$$R = \frac{{\Delta V}}{I}$$
(2)

The geometric factor, K, is expressed as

$$K = \pi \left( {\frac{{\left( {\frac{\text{AB}}{2}} \right)^{2} - \left( {\frac{\text{MN}}{2}} \right)^{2} }}{\text{MN}}} \right)$$
(3)

The aquifer protective capacity characterization is based on the values of the longitudinal unit conductance of the overburden rock units in the area. The longitudinal layer conductance (S) of the overburden at each VES station was obtained from Eq. (4). Table 1 gives the resistivity range with corresponding subsurface earth material (Ajayi and Hassan 1990) and Table 2 represents the protective capacity rating (Ogungbemi et al. 2013)

$$S = \mathop \sum \limits_{i = 1}^{n} \frac{hi}{\rho i}$$
(4)

where h is the saturated thickness of each layer and p is the layer resistivity

Table 1 Resistivity values of rock types (Ajayi and Hassan 1990)
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Table 2 Longitudinal conductance/protective capacity rating (Ogungbemi et al. 2013)
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The interpretation of the resistivity value for the n-layers was done for 60 VES points. The surfer11 computer programme was then used to produce iso-resistivity contour maps from the obtained data. The VES plots along the various profiles generate the geoelectric sections from where the resistivity variation with depth and thickness was obtained (Fig. 2).

Fig. 2
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Geoelectric Section a (VES curve A1), b (VES curve B1), c (VES curve C1), d (VES curve D1)

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Results and discussion

The results were summarized in a tabular form, giving information about the average layer resistivity, depth of each layer, thickness and the curve types (Table 3), while geoelectric section where the parameters in Table 3 were obtained is presented in Fig. 2.

Table 3 Layer resistivity, depth and thickness
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Iso-resistivity contour map for the first layer

The iso-resistivity contoured map for the first layer was generated at an interval of 50 Ωm (Fig. 3). The resistivity values range from 50 to 950 Ωm. The resistivity value shows that central portion of the area is a fadama loam, while sandy clay and gravels are located around southwest, northwest and north central parts of the area (Table 1).

Fig. 3
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Iso-resistivity contour map for the first layer

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Iso-resistivity contour map of the second layer

The iso-resistivity map for the second layer was generated at an interval of 50 Ωm (Fig. 4). Its resistivity values range from 0 to 1150 Ωm. From the map, it is observed that fresh laterite is scattered around the area and weathered laterite concentrates around northeast and central part of the area (Table 1). The weathered/fractured layer resistivity ranges from 65.84 to 454.9 Ωm, the depths of these weathered/fractured layers were found to be from 10.61 to 26.37 m and thickness 9.26 to 24.69 m, respectively. In a basement complex terrain, areas with overburden thickness of 15 m and above with fractured layer resistivity of < 1000 Ωm are good for groundwater development (Alhassan et al. 2015).

Fig. 4
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Iso-resistivity contour map of the second layer

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Iso-resistivity map of the third layer

The iso-resistivity map for the third layer was generated at an interval of 2000 Ωm (Fig. 5). The resistivity value ranges from 2000 to 2800 Ωm. The resistivity values on this map indicate the fresh basement.

Fig. 5
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Iso-resistivity contour map of the third layer

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Iso-resistivity map of the fractured basement produced shows the resistivity distribution across the study area which varies from 50 to 950 Ωm (Fig. 6). The Isopach map of the fractured basement is produced which gives thickness of the fractured basement across the area. The thickness varies between 1 and 23 m (Fig. 7). Isopach map of weathered layer is also produced, and it presents the thickness of the weathered basement within the study area (Fig. 8).

Fig. 6
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Iso-resistivity map of the fractured basement

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Fig. 7
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Isopach map of the fractured basement

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Fig. 8
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Isopach map of the weathered layer

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Isopach map of the overburden is produced. The map indicates that thick overburden is located around northwest, north central, north east, south east and south central portions of the area, while the shallow thickness is concentrated around extreme north, south west and central parts of the area (Fig. 9).

Fig. 9
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Isopach map of the overburden

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Aquifer protective capacities evaluation

The study revealed that the area is characterized with poor, weak and moderate protective capacities having a longitudinal conductance ranging from 0.01 to 0.30. The longitudinal conductance values obtained were used to produce the longitudinal contour map which shows the distribution across the area (Fig. 10). The highest longitudinal conductance was seen at VES B6 (0.30), and the lowest was seen at VES B2 (0.004).

Fig. 10
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Contour map of the longitudinal conductance distribution in the area

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Table 4 shows 16 VES stations recommended as aquifer potentials of the area. The highest groundwater yield is often obtained from a fractured aquiferous zone or a subsurface sequence that has a combination of a significantly thick and sandy weathered layer and fractured aquifer (Olorunfemi et al. 1999). A correlation of the nearby borehole log with the VES formation (Fig. 11) is in agreement.

Table 4 Recommended aquifer potentials of the study area
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Fig. 11
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a, b Correlation of VES data with nearby borehole log

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The curve types

The curve distribution (Fig. 12) indicates four types. The H-type is the dominant curve with 35% of the area The weathered/fractured layer in the H-curve type is usually characterized with low resistivity value made up of clayey or sandy clay, and it is usually water saturated and highly porous (Olorunfemi et al. 1999). The A-curve type consists 16% of the area, the HA curve type occupies 6%, while the AA occupies 3% of the study area.

Fig. 12
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Chart showing the curve distribution

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Conclusion

In the basement complex area, groundwater usually is found in a discontinuous aquifer. When defining the potentials of an aquifer formation, it comes with a tedious exercise due to the complexity properties of the basement rock (Alhassan et al. 2017). Therefore, the groundwater potentials of the study area were classified by employing the electrical resistivity of the fractured layer, the depth of the layer and the thickness of the layer. From the result, it is revealed that the subsurface consists of 3–4 lithologic units which are top layer, weathered layer, fractured layer and fresh basement. The curve types were identified as H, HA, A and AA. The geoelectric and geologic sections for the profile were produced. The aquifer potentials of the area show poor, weak and moderate protective capacities having longitudinal conductance ranging from 0.01 to 0.30 mho.

Sixteen VES stations were delineated as groundwater potentials of the study area, having fractured layer resistivity ranging from 65.84 to 454 Ωm. The depths of these layers are found ranging from 10.61 to 26.37 m and thickness ranging from 9.255 to 24.69 m. VES stations A10 and D9 are observed to have fine aquifer with depth ranging from 12.47 to 22.4 m and also thickness ranging from 10.68 to 21.2 m.

Recommendations

  • The VES stations delineated for groundwater potentials should be considered for water development

  • Areas with poor aquifer protective capacity should be avoided for sinking borehole to reduce leachates infiltration to the ground water